PROJECT SUMMARY/ABSTRACT The capacity of HIV envelope gp120 to cause immune suppression has been well documented in the literatures. These suppressive effects can be triggered by gp120 independent of virus infection. Recent data also demonstrate suppression of CD4 T cell activation by gp120 delivered via vaccine vectors. However, very little is known about the molecular mechanisms for this suppressive activity. In fact, the read-outs for gp120- mediated suppression have been based mainly on the standard T cell responses such as proliferation, cytotoxicity, cytokine production. The effects of gp120 on the upstream events essential for T cell activation are not at all clear. It is important to note that, upon engagement of CD4 and/or chemokine receptors, gp120 triggers unique gp120/CD4 supramolecular rearrangements and activation signals that involve many components of the T cell signaling machinery, but how the gp120-triggered clustering and signaling actually intersect with and alter the physiologic T cell receptor-mediated activation pathway remains unknown. The goal of this application is to investigate the consequences of CD4 T cell interaction with gp120 on the early events in T cell activation, i.e. the formation of immunological synapse and the subsequent signaling events. Our main hypothesis is that gp120 interferes with T cell receptor-mediated immunological synapse assembly and signaling to cause suppression of downstream T cell responses. To test this hypothesis, the synthetic glass-supported planar bilayer system will be utilized to mimic antigen-presenting cells bearing the cognate T cell receptor ligands (peptide-MHC complexes or anti-CD3), the costimulatory molecules, and gp120. Together with the state-of-the-art fluorescence microscopy, this experimental system enables acquisition of high-resolution and dynamic images of immunological synapse and its specific component. Hence, we will employ this innovative technology to explore the effects of gp120 interaction on two types of CD4 T cells, memory and naive, which display different immunological synapse characteristics and activation requirements. For each cell type, we will evaluate four parameters known to be critical for full CD4 T cell activation: a) immunological synapse assembly, morphology, and stability, b) T cell receptor-proximal signal activation, c) the immediate cellular responses (Ca2+ flux and CD69 upregulation), and d) the downstream effector functions (cytokine production and proliferation). HIV gp120 is an important target for anti-HIV antibodies. Since it has become clearer that induction of antibody responses is an indispensable element for HIV vaccines, we must understand better not only the immune responses to gp120 but also the immunosuppressive potential of this antigen, especially on CD4 T cells that provide help for the development and maintenance of both humoral and cellular responses. The data generated from this study will be valuable for designing gp120-based preventive and therapeutic strategies that are effective to block HIV infection and disease.